This invention relates to metallurgical technology, and, more particularly, to controlling the flow of a stream of molten metal.
Metallic articles can be fabricated in any of several ways, one of which is metal powder processing. In this approach, fine powder particles of the metallic alloy of interest are first formed. Then the proper quantity of the particulate or powdered metal is placed into a mold or container and compacted by hot or cold isostatic pressing, extrusion, or other means. This powder metallurgical approach has the important advantage that the microstructure of the product produced by powder consolidation is typically finer and more uniform than that produced by conventional techniques. In some instances the final product can be produced to virtually its final shape, so that little or no final machining is required. Final machining is expensive and wasteful of the alloying materials, and therefore the powder approach to article fabrication is often less expensive than conventional techniques.
The prerequisite to the use of powder fabrication technology is the ability to produce a "clean" powder of the required alloy composition on a commercial scale. (The term "clean" refers to a low level of particles of foreign matter in the metal.) Numerous techniques have been devised for powder production. In one common approach, a melt of the alloy of interest is formed, and a continuous stream of the alloy is produced from the melt. The stream is atomized by a gas jet or a spinning disk, producing solidified particles that are collected and graded for size. Particles that meet the size specifications are retained, and those that do not are remelted. The present invention finds application in the formation and control of the stream of metal that is drawn from the melt and directed to the atomization stage. More generally, it finds application in the formation and control of metal streams for use in other clean-metal production techniques.
The alloys of titanium are of particular interest in powder processing of aerospace components. These alloys are strong at low and intermediate temperatures, and much lighter than cobalt and nickel alloys that are used for higher temperature applications. However, molten titanium alloys are highly reactive with other materials, and can therefore be easily contaminated as they are melted and directed as a stream toward the atomization stage unless particular care is taken to avoid contamination.
Several approaches have been devised for the melting and formation of a stream of a reactive alloy such as a titanium alloy. In one such approach, the alloy is melted in a cold hearth by induction heating. The alloy stream is extracted through the bottom of the hearth and directed toward the atomization apparatus. The stream may be directed simply by allowing it to free fall under the influence of gravity. To prevent excessive cooling of the stream as it falls, electrical resistance heating coils have been placed around a ceramic nozzle liner through which the stream passes, as described for example in U.S. Pat. No. 3,604,598. Another approach is to place an induction coil around the volume through which the stream falls, both to heat the stream and to control its diameter, as described for example in U.S. Pat. No. 4,762,553. These and similar techniques have not proved commercially acceptable for the control of a stream of a reactive titanium alloy for a variety of reasons.
There therefore exists a need for an improved approach to the formation and control of a stream of a metal, and particularly for reactive metals such as titanium alloys. The present invention fulfills this need, and further provides related advantages.